Pregnant mice (CD-1 strain) were exposed to 2.45 GHz microwave radiation at a power density level of 30 mW/cm.squared. At exposure to 30 mW/cm squared (SAR is approximately equal to 32 mW/g) during days 1-6 a significant decrease in implantation sites per litter and average fetal weight was observed. Exposure to 30 mW/cm squared during days 6-15 resulted in a slight increase in the number of malformed fetuses but was not statistically significant as obtained in a previous experiment. This repeat of previous work again indicates that the threshold for teratogenic effects in the CD-1 mouse strain is approximately 30 mW/cm squared. Peripheral blood and bone marrow samples were obtained on day 18 of pregnancy from the dams. Total leukocyte and differential leukocyte counts of peripheral blood samples were not affected. No effects were noted in either the erythroid or myeloid mitotic index of bone marrow samples. Studies specifically designed to determine the effects on development preimplantation embryos were carried out at power densities of 9 mW/cm squared and 19 mW/cm squared on either day 2 or 3 of pregnancy. Heat stress due to exposure caused stunted development of embryos, but no remarkable effects were found on the development of preimplantation embryos.